Generally speaking, a bearing comprises an inner ring and an outer ring adapted to rotate around a rotation axis, one with respect to the other. In a plain bearing, the two rings are in sliding contact. In a rolling bearing, several rolling bodies are installed between the two rings. These rolling bodies can be balls, rollers or needles. Thus, a rolling bearing can be, for instance, a ball bearing, a roller bearing or a needle bearing.
In the field of bearings, it is known to use a tachometer in order to determine the rotation speed of a shaft supported by a bearing with respect to a supporting member, such as the casing of an electric machine. As explained in EP-A-1 933 155, one can use an encoder washer with magnetic poles fast in rotation with a rotating ring of a bearing, and one or several sensors distributed around the encoder washer or located in front of it. An air gap is provided between the encoder washer and the or each sensor. Detection of a rotation parameter of the encoder washer occurs through this air gap, thanks to the variation of a magnetic field generated by the encoder washer.
It is known from FR-A-2 698 421 to use an annular part forming a sensor support member, for a sensor designed to cooperate with a permanent magnet.
A key point for an efficient detection of a rotation parameter is that the magnetic field generated by the encoder washer in the vicinity of the sensor should not be disturbed by a surrounding magnetic field. This cannot be guaranteed with the devices of the prior art, insofar as the air gap between the encoder washer and the or each sensor is not efficiently protected against magnetic perturbations. This is all the more important that some applications of a rotation detection occur in a strongly perturbated magnetic environment, such as in the vicinity of the rotor and stator of an electric machine where intense magnetic fields are generated.